1. Field of the Invention
This invention relates to a differential gear that allows a rotational difference between the left-hand and right-hand driving wheels or between the front and rear driving wheels of a vehicle.
2. Description of the Prior Art
In the past, the differential gear used for the driving force transmission system of vehicles has been a system that allows a rotational difference between the left-hand and right-hand driving wheels when a vehicle travels on curves or the rotational difference between the front and rear driving wheels of a four-wheel drive vehicle. However, when only one driving wheel runs on a surface with an extremely low coefficient of friction such as snow or sand, this wheel races and the whole driving force is lost with the result that the vehicle cannot escape from such a surface.
If the load on the inner wheel is substantially reduced by centrifugal force when the vehicle travels on a curve at high speed, the driving force to negotiate the curve at high speed is easily lost. To make up for such a drawback, some vehicles are provided with a differential limitation mechanism, for example, of a clutch disk crimping type, thereby limiting the differential rotation of each driving wheel under a specified condition. However, this type indicates a sudden differential limitation effect with the increase of rotational difference and mutually binds individual wheels when no driving force is input from the engine side. This makes it difficult to combine in a system in which independency is required for the rotation of each wheel such as in the antilock brake system.
For this reason, a differential limitation mechanism of rotation speed sensitizing type using a viscous coupling is often used of late to maintain independency for the rotation of each driving wheel limiting the differential rotation under a specified driving condition. The viscous coupling is a type of a viscous clutch which transmits torque by means of the shear resistance of a viscous fluid (e.g., silicone oil), so a smooth differential imitation effect can be obtained in accordance with each rotational difference.
A differential gear having a mechanism to limit the differential rotation only at driving without limiting each driving wheel at non-driving, for example, a torque sensitizing type combined with a worm gear is already known and described in Provisional Publication No. 271926/92 of the Japanese Patent Application Official Gazette. In this type, a pair of screw-shaped worms that can be rotated independently of each other on the same shaft are provided in a gear with multiple worm wheels having a rotary shaft perpendicular to it. When each worm is rotated, the worm wheels can rotate smoothly. However, if the worms are rotated from the worm wheel side, this is difficult. This character is peculiar to the worm gear. Accordingly, this type is provided with the character that differential rotation and a differential limitation effect are obtained in accordance with each condition.
One drawback of the differential gear with a differential limitation mechanism of the rotation speed sensitizing type, represented by the viscous coupling, however, is that the torque transmissibility depends on the viscosity of a fluid, and this viscosity varies with temperature changes and cannot provide a stable differential limitation effect at all times. Another problem with this type is that a time lag occurs between the occurrence of a rotational difference and the execution of differential limitation, thereby making it impossible to instantly cope with running operation changes.
On the other hand, the differential gear using worm gears performs the differential limitation mechanically, so that a stable differential limitation effect can be obtained. Its drawback, however, is that its structure is complicated with numerous parts whose working and assembly require high precision, and the whole system size is large compared with the allowable torque.